One interesting use of the Aim At constraint I found on the A:M site was to constrain billboarded images (for example, images of leaves attached to trees) to Aim At the camera. Since the image is always pointing at the camera, it always looks 3D. It's a pretty effective trick.

It also reminded me of this article, which uses the Orient Like constraint to constrain a complex chain of bones to a less complex chain. The end result is that you can control a complex chain of 12 bones with only 4 bones:
I thought it was particularly clever.